Audio Research - should I replace the valves?

I have a AR Ref 110 power amp and AR Ref 3 preamp. Both have done about 3,800 hours. They both work but I wonder if should be replacing the valves.

How can you tell if they’ve had it? Does the sound quality deteriorate & what happens if you don’t replace them?

i don’t think they sounds as good as they did but no pops or whistles or any other strange noises etc - just a bit flat sounding. But perhaps this is just my ears?

pp